1. Field of the Invention
The present invention relates to partial response maximum likelihood (PRML) methods and systems and to the use of tape drives and magnetic tapes to store data.
2. Background Art
In a traditional magnetic storage medium, a read circuit detects flux reversals to determine a data signal. Peak detection is used to interpret the information present in the data signal. As data areal density increases, the peaks get smaller and smaller relative to the background noise, and accordingly, get more and more difficult to detect. A technique used to allow further increases in data areal density that addresses difficulties associated with peak detection is partial response maximum likelihood (PRML).
PRML does not attempt to detect individual peaks in the way that the traditional peak detection techniques do. PRML uses digital signal processing to analyze the analog data signal from the read circuit to determine the most likely pattern of flux reversals. That is, PRML determines the most likely data stream based on the partial response observed in the analog data signal from the read circuit. PRML techniques have been quite successful in allowing the continued increase in areal data density for magnetic storage applications.
The use of tape drives and magnetic tapes to store data has become widespread. Tape drives have many advantages for certain storage applications in that they are able to meet the capacity, performance and reliability needs of these applications at an acceptable cost. A problem that occurs in tape drives is that the head to tape interface is not always consistent. The distance between the head and the tape can vary. In addition, the uniformity of the magnetic coating on the tape varies along the length of the tape. Further, head wear and tape wear result in further inconsistencies in the head to tape interface.
PRML approaches have been used in tape drive applications. The inconsistent head to tape interface causes error rates to increase dramatically in tape drive applications that rely on traditional PRML. To address the problem of increasing error rates, attempts have been made to employ parallel equalization and detection channels for each track of the tape. In this way, each of the parallel equalization/detection channels may be tuned to a different response to account for the inconsistent head to tape interface. Accordingly, the most likely data stream for a particular track is determined based on the observation of the several equalization/detection channels.
The equalization/detection capability for multiple parallel tracks may be provided in a single application specific integrated circuit (ASIC). The ASIC receives an input signal from the read circuit for each track being read in parallel. The input signals undergo common input signal conditioning. For each track input signal, there are multiple equalization/detection channels employed in parallel, each tuned to a different response, to account for inconsistent tape to head interfacing.
This approach of providing multiple parallel equalization and detection channels for each tape track does address the problem of errors due to inconsistent head to tape interfacing, however, as the number of parallel tracks on a tape increases, the difficulty in terms of cost and physical implementation for the application specific integrated circuit (ASIC) that performs that detection goes up significantly. Further, demands to reduce overall physical space for the tape drive components also add to the difficulty of providing multiple parallel equalization/detection channels for each tape track.
For the foregoing reasons, there is a need for an improved PRML method and tape drive that address head to tape interface inconsistencies and also address cost and physical implementation issues.